A CENTURY'S PROGRESS IN PHYSICS 379 



The close connection between electrodynamics and the 

 principle of relativity is obvious from the fact that both 

 lead to the same time and space transformations. Fur- 

 thermore L. Page (37, 169, 1914) has shown that the 

 electrodynamic equations can be derived exactly and in 

 their entirety from nothing more than the kinematics of 

 relativity and the assumption that every element of 

 charge is a center of uniformly diverging lines of force. 

 Hence it may safely be asserted that no purely electro- 

 magnetic phenomenon can ever come into contradiction 

 with this principle. The simplicity thus introduced into 

 the solution of a certain class of problems is enormous. 

 As an example consider the question as to whether a mov- 

 ing star is retarded by the reaction of its own radiation. 

 This purely electrodynamical problem is of such com- 

 plexity that attempts to solve it have led to some contro- 

 versy among mathematical physicists. The principle of 

 relativity tells us without recourse to analysis that no 

 retardation can exist. 



Throughout the nineteenth century the ether has 

 played a fundamental part in all important physical 

 theories of light and electromagnetism. But if it is not 

 possible for experiment to detect even the state of 

 motion of the ether, why postulate the existence of such a 

 medium I If it does not possess the most fundamental 

 characteristic of matter, how can it possess such derived 

 properties as density and elasticity, properties which 

 any conceivable mechanical medium must have in order 

 to transmit transverse vibrations? The relativist does 

 not deny the existence of an ether. To him the question 

 has no more meaning than if -he were asked to express an 

 opinion as to the reality of parallels of latitude on the 

 earth's surface. As a convenient medium of expression 

 in describing certain phenomena the ether has justified 

 much of the use which has been made of it.^ But to 

 attribute to it a degree of substantiality for which there 

 is no warrant in experiment, is to change it from an aid 

 into an obstacle to the progress of science. From the 

 relativist point of view the distinction is very sharp 

 between those motions of charged particles which are 

 experimentally observable, and such geometrical conven- 

 tions as electromagnetic fields, or analytical symbols as 



